1. Field of the Invention
The present invention relates to a securing hook for mounting and fixing a member to be secured such as a printed circuit board, a case cover part, or the like, which is equipped in an electric appliance, at a given position within the main body of the electric appliance.
2. Description of the Related Prior Art
In an electric appliance, it is necessary that a printed circuit board with an integrated circuit and electric elements mounted thereon is mounted and fixed at a given position within the main body of the electric appliance. As a fixing structure used to mount such printed circuit board, for example, there are available a fixing structure of a type that there are opened up in a printed circuit board through holes through which screws for fixing the printed circuit board can be inserted, and the printed circuit board is screwed onto bosses provided in the main body of the electric appliance by inserting such screws through the through holes; and, a fixing structure which uses special metal members developed specially for fixing such printed circuit board. Also, there is widely used a method for fixing a printed circuit board using a securing hook molded of resin, because this method allows the printed circuit board to be fixed easily and is advantageous in the cost of parts required.
Here, FIGS. 4A-4B, 5A-5C and 6 respectively show a conventional securing hook which is formed of resin material. In particular, FIG. 4A is a perspective view of a securing block 300 which is formed of resin material according to an outsert molding method or the like for use in a metal chassis (not shown) or the like. And, FIG. 4B is a perspective view of the securing block 300 and a member to be secured 400 such as a printed circuit board or the like, showing a state thereof in which the member to be secured 400 is mounted on and fixed to the securing block 300.
Also, FIGS. 5A to 5C respectively show a process for mounting and fixing the member to be secured 400 onto the securing block 300, using a section view taken along the line A--A shown in FIG. 4A. Further, FIG. 6 shows a state in which a force to remove the member to be secured 400 from the securing block 300 is applied to the member to be secured 400 fixed to the securing block 300.
Now, description will be given below in detail of the structure of the conventional securing hook with reference to FIGS. 4A, 4B, 5A-5C and 6.
The securing block 300 comprises a receive part 320 on which the member to be secured 400 is to be placed, and a securing hook body 310 which is used to fix the member to be secured 400 onto the receive part 320, while the receive part 320 and securing hook body 310 are formed of resin material as an integrally united body. By the way, the illustrated receive part 320 is formed in a plane shape. And, conventionally, there is also known a method in which, on the plane of the receive part 320, a positioning projection is formed of the same resin material as the receive part 320; and, in the member to be secured 400, there is formed a hole which can be engaged with the positioning projection, whereby the member to be secured 400 can be positioned when mounting and fixing the member to be secured 400.
The securing hook body 310 is composed of a straight portion 311 which extends upwardly in a substantially straight manner, and a hook portion 312 which is disposed in the leading end portion of the straight portion 311. And, in the hook portion 312, there are formed a linear-shaped sliding contact surface 313 which is located on the side of the hook portion 312 where the member to be secured can be secured and also which is inclined downward at a given angle in a direction to move away from the straight portion 311 and, a securing portion 314 which can be engaged with the upper surface of the member to be secured 400 placed on the receive part 320 to thereby fix the member to be secured 400 onto the receive part 320. The securing portion 314 is so formed as to have a surface substantially parallel to the upper surface of the receive part 320 (that is, a surface intersecting substantially at right angles to the extending direction of the straight portion 311).
Now, description will be given below in detail of a process in which the member to be secured 400 such as a printed circuit board or the like is pushed into the above-structured securing block 300 by an assembler and is then fixed onto the receive part 320, with reference to FIGS. 5A to 5C.
Specifically, FIG. 5A shows a state in which, in order to fix the member to be secured 400 onto the securing block 300, the assembler has brought part of one of the sides of the member to be secured 400 into contact with a given position (P1) of the sliding surface 313 of the hook portion 312. In this state, when the assembler tries to push down the member to be secured 400 toward the receive part 320, the member to be secured 400 is pushed at a pressure angle (.alpha.1) that can be determined by the angle of inclination of the sliding contact surface 313 with respect to a direction in which the member to be secured 400 is pushed.
And, when the assembler, while keeping part of one side of the member to be secured 400 in sliding contact with the sliding contact surface 313, pushes down the member to be secured 400 in the direction of the receive part 320 to thereby move the member to be secured 400 down to a second position (P2) which, as shown in FIG. 5(b) , is present on the sliding contact surface 313 and is located downwardly of the given position (P1) , due to the action of the pushing force, the straight portion 311 of the securing hook body 310 is flexed in a direction where it moves apart from the receive part 320 (in FIG. 5B, in the right direction). In this operation, as the straight portion 311 is flexed, there is generated a flexure angle to thereby change the inclination angle of the sliding contact surface 313 with respect to the pushing direction of the member to be secured 400. Accordingly, as shown in FIG. 5B, a pressure angle (.alpha.2) at the position (P2) becomes greater than the pressure angle (.alpha.1) at the position (P1).
Now, description will be given below in detail of the variations in the pressure angle with reference to FIG. 7.
That is, in FIG. 7, in a state where a member A having a length of 1 and having the same cross section is held in a cantilever manner with one end al thereof fixed, assuming that the free end a2 of the member A is moved by a distance of .delta. by applying a force p to the free end a2, the relation shown in the following equation holds (E: modulus of longitudinal elasticity, I: Section secondary moment). EQU .delta.=p1.sup.3 /3EI EQU .theta.=P1.sup.2 /2EI
From the above relation equation, the variation amount of the flexure angle .theta. of the member A can be expressed by the following equation: EQU .theta.=(3/2I).delta..
Therefore, when taking the above relation into consideration, as shown in FIGS. 5A and 5B, according as the sliding contact position between the member to be secured 400 and sliding contact surface 313 varies as a result of the member to be secured 400 being pushed and moved, the straight portion 311 increases in the flexure amount .delta. gradually. With the increase in the flexure amount .delta., the flexure angle also increases. And, as the flexure angle increases, the pressure angle also increases. Let us express the variations of the sliding contact position between the two elements in a numerical manner. That is, if the initial pressure angle at the position (P1) is expressed as (.alpha.1), then the pressure angle (.alpha.2) at the position (P2), namely, the flexure amount .delta. can be expressed as follows: EQU (.alpha.2)=(.alpha.1)+(3/2I).delta.
When the assembler has pushed the member to be secured 400 down to a position where the member to be secured 400 can be put on the receive part 320 through the above-mentioned pushing process, due to the restitutive force of the straight portion 311 of the securing hook body 310, the flexure of the straight portion 311 is removed, so that the hook portion 312 is allowed to move toward its original position. At the then time, the securing portion 314 of the securing hook body 310 is engaged with the upper surface of the member to be secured 400 put on the receive part 320. This restricts the movement of the member to be secured 400 in the upward direction thereof, which makes it possible to secure and fix the member to be secured 400 onto the receive part 320.
On the other hand, as shown in FIG. 6, when a force to move the member to be secured 400 away from the securing block 300 is applied to the member to be secured 400 which is secured and fixed to the securing block 300, the straight portion 311 of the securing hook body 310 is flexed in a direction to move away from the receive part 320, with the result that the hook portion 312 is spread out.
In other words, when a force to move the member to be secured 400 away from the securing block 300 is applied to the member to be secured 400 which is secured and fixed to the securing block 300, the hook portion 312 is spread out, which, unfavorably, allows the member to be secured 400 to be removed easily.
In other words, in the above-described conventional securing hook 310, in addition to the above-mentioned problem, as the assembler pushes the member to be secured 400 in the downward direction, that is, in the direction of the receive part 320, the pressure angle thereof increases gradually; and, as the pressure angle thereof increases gradually, it is necessary for the assembler to increase gradually the pushing force to push the member to be secured 400. This requires the assembler to have skill for pushing the member to be secured 400 when assembling the securing hook 310.